Accurately determining water depth is a crucial aspect of navigating littoral waters. The measurement of water depths, also known as bathymetry, is also useful in oil and gas exploration, pipeline and cable laying, recreational water activities and in environmental fields, such as in the study of marine ecosystems and the effects of climate change. Historically, nautical charts for maritime transportation were prepared by simply looking into the water or by lowering a weight on a rope into the water and then noting the rope length when the weight no longer pulled on the rope. More recently, echo sounding has been used to determine the depth of the sea floor by measuring the time for a SONAR (SOund NAvigation and Ranging) signal to travel to the sea floor, bounce off and return to the hydrographic survey ship above. In addition to SONAR, which uses sound waves, LIDAR (LIght Detection And Ranging) is also used for bathymetry mapping. However, performing bathymetric surveys using ships is quite expensive and time consuming. In addition, storms, currents and erosion change the underwater topography, so nautical charts must be updated. The amount of global littoral waters is simply too extensive to be surveyed on a regular basis. In some remote areas, the most recent nautical charts are hundreds of years old.
Attempts have been made to use earth observation sensors on satellites to perform bathymetry. The satellite measures radiation from the sun that is reflected from the earth. Water depth can be determined if the reflection of the sea bottom contributes a detectable part to the radiation measured by the sensor. Radiation at multiple wavelengths that emanates from a point on the water surface is measured at the satellite. Then an attempt is made to determine the water depth based on isolating the bottom reflection from other factors that scatter and absorb the sun's radiation, such as atmospheric molecules, pure water, substances in the water, the type of bottom surface, and the water surface roughness. Because these other factors change over time and over different coastal areas, previous attempts to determine an accurate depth at each survey location have failed. Only the relative depths in a small area have been reliably determined. For example, as the turbidity of the water gradually changes with location, existing bathymetric methods using satellite data show a change in depth even where the sea floor is flat. Likewise, existing methods can indicate that the depth at two locations is the same even where the actual depths are different because the water at the two locations exhibits differing turbidities.
A method is sought that uses satellite data to determine accurate water depths despite the effects of water turbidities that vary over time and location.